The characterisation of subsurface strata is important for, among other things, identifying, accessing and managing reservoirs. The depths and orientations of such strata can be determined, for example, by seismic surveying. This is generally performed by imparting energy to the earth at one or more source locations, for example, by way of controlled explosion, mechanical input, vibration mechanism etc. Return energy is then measured at surface receiver locations, marine receiver locations at varying distances and azimuths from the source location. The travel time of energy from source to receiver, via reflections and refractions from interfaces of subsurface strata, indicates the depth and orientation of the strata.
Passive or microseismic measurements can be characterized as a variant of seismics. In conventional seismic explorations a seismic source placed at a predetermined location, such as one or more airguns, vibrators or explosives, is activated and generates sufficient acoustic energy to cause acoustic waves to travel through the Earth. Reflected or refracted parts of this energy are then recorded by seismic receivers such as hydrophones and geophones. The terms passive seismic and microseismic are used interchangeably herein.
In microseismic monitoring there is no actively controlled and triggered seismic source at a known location. The seismic energy is generated through so-called microseismic events caused by subterranean shifts and changes that at least partially give rise to acoustic waves which in turn can be recorded using suitable receivers. Although the microseismic events may be a consequence of human activity disturbing the subterranean rock, the events are quite different from operation of equipment provided as an active seismic source.
Microseismic monitoring has become a standard technique to monitor fracture propagation during hydraulic fracture stimulation.
Reservoir and completion quality are parameters used to define shale reservoirs. Shales are highly textural anisotropic rocks with varying diagenetic histories and mineralogical contents, which often lead to complex fracture networks. In shales, dissimilarities in fracture evolution within a short interval along lateral treatment wells are commonly observed. Factors responsible for the variability are still poorly understood. Besides local stress variations and rock properties, pre-existing faults and other zones of weakness of the rock are assumed to play a major role. Pre-existing faults can result in undesired reservoir stimulation by, for instance, guiding hydraulic fractures to water-bearing zones. Consequently, in order to properly control/manage a fracturing process, detailed knowledge about the fault network is very important, if not essential.
However, rock fabric is at the limit of, or even below, 3D reflection seismic resolution, which makes mapping the fault network/rock fabric a challenging task. For purposes of this disclosure, the term “rock fabric” is used to describe pre-existing small scale discontinuities and zones of weakness of the rock/formation and the term “fracture(s)” is used to describe hydraulically induced fractures produced through stimulation of the rock/formation; such as in a hydraulic fracturing process where fracturing fluids are pumped into a wellbore to fracture the rock/formation surrounding the wellbore.